Astigmatic difference correcting method for optical head and apparatus therefor

ABSTRACT

According to an astigmatic difference correcting method for an optical head, a rotational optical member which is arranged between a first laser beam source having an astigmatic difference and a collimator lens to be rotatable around an optical path of a laser beam of a first wavelength emitted from the first laser beam source is rotated and adjusted. Rotation and adjustment, of the rotational optical member is caused to set a minimum difference between focal positions in an X-axis direction and in a Y-axis direction which are perpendicular to the optical path of the laser beam. The laser beam in which the difference between the focusing offset positions is minimum is irradiated on an optical disk having a recording density having a bit length not more than half a laser wavelength.

BACKGROUND OF THE INVENTION

The present invention relates to a method of improving an optical diskdevice, for a ROM disk, a writing once disk, or an erasable disk forhigh-density recording, in which a focusing offset for the bestreproduction signal is different from a focusing offset for the besttracking error signal and, more particularly, to an astigmaticdifference correcting method for an optical head used in an optical diskdevice for high-density recording or the like and an apparatus therefor.

An optical system for a conventional optical head has no apparatus forcorrecting an astigmatic difference of a semiconductor laser. In theoptical system, a focusing offset for the best reproduction signal isgenerally different from a focusing offset for the best tracking errorsignal.

FIGS. 15 and 16 respectively show the best RF signal and the besttracking error signal when the levels of a digital reproduction (RF)signal which forms an eye pattern and a tracking error (TE) signal arechanged with a change in focusing offset. As is apparent from FIGS. 15and 16, when a focused beam has an astigmatic difference due to a changein focusing offset, a modulation transfer function (to be referred to asan MTF hereinafter) in the tangential direction of an optical diskincreases. That is, it is understood that the best RF signals and besttracking error signals of beams focused in the tangential and radialdirections of the disk are different conditions and these situations aredepending on focusing offsets. More specifically, when an astigmaticdifference occurs due to a focusing position of a focused beam, the beambecomes an elliptic beam, and the lengths of the major and minor axesare different from each other. For this reason, MTFs are different inthe tangential and radial directions of the disk which are perpendicularto each other. An RF signal is maximum when the beam is focused in thetangential direction of the disk, and the best tracking error signal canbe obtained when the beam is focused in the radial direction of thedisk. In this case, when the maximum and minimum outputs of areproduction signal from the optical disk are represented by A and B,respectively, an MTF is expressed by MTF=(A-B)/(A+B).

It is important that a tracking servo operation is stabilized and a goodRF signal is obtained in an optical head designed to have a highrecording density and a high transfer rate.

In an optical head, the best focusing offset position obtained when atracking error signal is maximum is different from a focusing offsetposition obtained when an RF signal is maximum. Therefore, although atracking servo operation is stabilized when the tracking error signal isbest, a good signal cannot be reproduced when focusing position is seton good condition for tracking in MTF. In contrast to this, when afocusing offset is set to obtain the best RF signal, a trackingoperation is disadvantageously performed. A serious problem in which afocusing offset for the best tracking error signal is different from afocusing offset for the best RF signal is supposed to be posed by theastigmatic difference of a focused beam.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an astigmaticdifference adjusting method for an optical head, in which a focusingoffset for the best tracking error signal can be set equal to a focusingoffset for the best RF signal by making it possible to easily adjust anastigmatic difference of the optical head.

In order to achieve the above object, referring to the presentinvention, there is provided an astigmatic difference correcting methodfor an optical head, comprising the steps of rotating and adjusting arotational optical member which is arranged between a first laser beamsource having an astigmatic difference and a collimator lens to berotatable around an optical path of a laser beam of a first wavelengthemitted from the first laser beam source, causing rotation andadjustment of the rotational optical member to set a minimum differencebetween focal positions in an X-axis direction and in a Y-axis directionwhich are perpendicular to the optical path of the laser beam, andirradiating the laser beam in which the difference between the focusingoffset positions is minimum on an optical disk having a recordingdensity having a bit length not more than half a laser wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exploded view showing a laser pen having an astigmaticdifference correcting apparatus according to an embodiment of thepresent invention;

FIG. 1B is a front view showing a collimator lens holder shown in FIG.1A;

FIG. 2 is an assembly drawing showing the laser pen shown in FIG. 1A;

FIG. 3 is an exploded perspective view showing an optical head havingthe astigmatic difference correcting apparatus shown in FIG. 1A;

FIG. 4 is a view showing optical paths in astigmatic differencecorrection performed by a cylindrical lens;

FIG. 5 is a graph showing changes in focal length on X and Y planescaused by rotation of a cylindrical lens;

FIG. 6 is a view for explaining astigmatic difference correctionperformed by an inclined plate glass;

FIG. 7 is a view showing optical paths in astigmatic differencecorrection performed by the inclined plate glass;

FIG. 8 is a graph showing a change in astigmatic difference caused bythe inclined plate glass;

FIG. 9 is a view showing an astigmatic difference adjusting system foran optical head;

FIG. 10A is a chart showing a signal waveform when an astigmaticdifference is large;

FIG. 10B is a chart showing a signal waveform when an astigmaticdifference is corrected;

FIG. 11 is a view showing the entire arrangement of an optical headhaving an astigmatic difference correcting apparatus according toanother embodiment of the present invention;

FIG. 12 is a perspective view showing main part for explainingrotational driving of an astigmatic difference correcting optical partshown in FIG. 11;

FIG. 13 is a graph showing the spectral characteristics of a dichroicmirror shown in FIG. 11;

FIG. 14 is a view showing another example of the dichroic mirror shownin FIG. 11;

FIG. 15 is a graph showing a conventional RF signal which is notsubjected to astigmatic difference correction; and

FIG. 16 is a graph showing a conventional tracking error signal which isnot subjected to astigmatic difference correction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings.

FIG. 1A shows an astigmatic difference correcting apparatus according tothe first embodiment of the present invention, and FIG. 1B shows acollimator lens holder shown in FIG. 1A.

Referring to FIG. 1A, an inclined plate glass 1 is arranged between asemiconductor laser 2 having an astigmatic difference and a collimatorlens 3. During astigmatic difference correction, the inclined plateglass 1 is rotated (rotational angle=φ) around an optical axis to adjustthe inclined plate glass 1. FIG. 1A shows the arrangement of a laser penhaving a corrected astigmatic difference.

A collimator lens holder 4 is obtained such that slits 5 in an axialdirection are formed in a cylindrical member having an elliptic flangeportion 4a formed at one end of the collimator lens holder 4 on thelaser 2 side. The collimator lens 3 is fitted in the distal end portionof the collimator lens holder 4 at an accuracy at which the collimatorlens 3 is pressed against the collimator lens holder 4. As shown in FIG.1B, the slits 5 are formed in two opposing portions of the cylindricalmember.

Since the slit 5 has mechanical rigidity, the collimator lens 3 is slidalong the optical axis to make it possible to adjust the collimator lens3. With respect to one of the two slits 5, a large rotation adjustinghole 6 is formed in a portion where a laser is to be attached. The hole6 is used as an adjusting hole for adjusting the rotation of theinclined plate glass 1 held by an inclined plate glass holder 15.

A laser attachment plate 7 is brought into contact with the flangeportion 4a of the collimator lens holder 4, and the laser 2 and a laserholding plate 8 are attached to the plate 7. A high-frequencysuperposing module 9 is attached to the plate 7 through the holdingplate 8. The structure assembled as described above is shown in FIG. 2.A beam emitted from the laser 2 is converted into a parallel beam by thecollimator lens 3 held at the distal end of the collimator lens holder4.

FIG. 3 shows the details of assembling of the laser pen and adjustmentof the inclined plate glass 1. The parallel beam from the collimatorlens 3 is deflected at 90° by a mirror 10 and guided to an objectivelens actuator 11 of the optical head. In this case, the focused state ofthe parallel beam in a Z-axis direction is measured by a focused beamanalyzer 13 having an objective lens 12 having a numerical aperture(NA)=0.9.

This analyzer 13 performs stepping on a 0.1-μm order, and can measurethe focused beam as the sectional views (X and Y cuts) of the focusedbeam on X and Y planes in focused beam detection by the optical head. Atthis time, when a light amount at an in-focus point of the focused beamis set to be 100%, a position at which a focused beam size in a lightamount of e⁻² (13.5%) is minimum is set on each of the X and Y planes,and these positions are set as focal points on the X and Y planes. Adifference L between the focal points in two orthogonal directions isset as an astigmatic difference.

The inclined plate glass 1 is rotated and adjusted while the astigmaticdifference is observed. This adjustment is performed such that a lever14 having a projection is fitted in a groove 15a formed in the inclinedplate glass holder 15.

FIG. 4 shows a case wherein a cylindrical lens 42 is used in place ofthe inclined plate glass 1. Referring to FIG. 4, the cylindrical lens 42is arranged between the laser 2 and the collimator lens 3. With respectto X-axis and Y-axis serving as coordinate axes in FIG. 3, beam trackson the X-axis and Y-axis planes are respectively shown in upper andlower sides in FIG. 3. The cylindrical lens 42 which causes a change inastigmatic difference when the cylindrical lens 42 is rotated about theoptical axis (Z-axis) has no power on the X plane but has a power on theY plane. The astigmatic difference of the laser 2 is corrected byrotating the cylindrical lens 42 about the optical axis.

The focal length of the cylindrical lens 42 is represented by f2; thefocal length of the collimator lens 3, f1; the focal length of theobjective lens 12, f0; and the distance between the laser 2 and thecylindrical lens 42, e. In this case, a correctable astigmaticdifference A is expressed by equations (1) and (2).

    Δ.sub.0 =f1/f2.e (rotational angle φ about optical axis=0°)                                           (1)

    Δ=Δ.sub.0.cos(2φ)                          (2)

The difference between the focal positions (focal points) of thecollimator lens 3 on the X and Y planes is an astigmatic difference. Theastigmatic difference (as) of the objective lens 12 is expressed byequation (3).

    as=(f0/f1).sup.2.Δ                                   (3)

A cylindrical lens having a large focal length and a curvature radius of5,000 mm or more and arranged in a collimated beam can be rotated aboutthe optical axis in the same manner as described above to adjust thecylindrical lens. The cylindrical lens is rotated to calculate thereciprocals of the focal lengths in the X- and Y-axes, thereby obtainingthe same characteristics as shown in FIG. 5. When the cylindrical lensis rotated about the optical axis, and a focal length in each axialdirection is represented by f, a relation shown as equation (4) isobtained.

    1/f=1/f2.cos(φ)                                        (4)

The reciprocal of a change in focal length when the cylindrical lens isrotated about the optical axis is shown in FIG. 5.

FIGS. 6 and 7 show a case wherein the inclined plate glass is rotated.FIG. 7 shows a case wherein a coordinate system obtained by arrangingthe respective optical parts shown in FIG. 6 is set. The beam track onthe upper side shows the X plane, and the beam track on the lower sideshows the Y plane. A change in astigmatic difference Δ when the inclinedplate glass 1 is rotated about the Z axis is shown in FIG. 8. In thiscase, the same relation as equation (2) is obtained.

An apparatus for correcting the astigmatic difference between the laserof an optical head and a compound prism 91 is shown in FIG. 9.

Referring to FIG. 9, an expensive objective lens having a numericalaperture of 0.9 is not used, a focused beam emitted from an objectivelens actuator 92 through a laser pen 96, the compound prism 91, and anoptical path direction changing unit 97 is input to an objective lens 93having a numerical aperture almost equal to that of the lens used in theoptical head. The objective lens actuator 92 for measuring an astigmaticdifference is wobbled in the condition of setting a cover glass having athickness corresponding to the cassette coating of a disk. The focusedbeam which is changed by the astigmatic difference is input to afour-division sensor 94. Two addition signals obtained by adding eachtwo outputs from the four-division sensor 94 are input to a differentialamplifier 95, thereby obtaining a waveform representing an astigmaticfocusing error signal. The waveforms of the signals obtained by thedifferential amplifier 95 are shown in FIGS. 10A and 10B, respectively.FIG. 10A shows the waveform of the signal obtained when an astigmaticdifference is large, and FIG. 10B shows the waveform of the signalobtained when an astigmatic difference is corrected. An adjustment lever98 operates to decrease an interval T between positive and negativepeaks of each waveform, thereby rotating and adjusting the cylindricallens or the inclined plate glass.

FIG. 11 shows an optical head having an astigmatic difference correctingapparatus according to the second embodiment of the present invention.As shown in FIG. 11, the astigmatic difference correcting apparatuscomprises an adjusting unit 110 for actually performing astigmaticdifference correction, an optical path direction changing unit 120 forchanging the directions of the optical paths of a laser beam emittedfrom the adjusting unit 110 and a beam reflected by a optical disk 150for high-density recording, a spectral unit 130 for causing a laser beamfrom the optical path direction changing unit to be incident on andemerge from the optical disk 150, and an error detector 140.

In this case, an optical disk for high-density recording used in thepresent invention is an optical disk having a bit length which is half alaser wavelength. More specifically, an optical disk having a highrecording line density in which a bit length is 2.4 times the density ofa currently available compact disk (CD) having 0.8 μm/bit and is 1.02times a laser wavelength (λ=0.78 μm) is called an optical disk forhigh-density recording.

The adjusting unit 110 comprises an optical part 101 consisting of aninclined transparent plate glass, a laser beam source 102 serving as alight source, a collimator lens 103, and a collimator holder 105 forstoring these components. The collimator holder 105 has a cylindricalshape, and a slit 106 is formed from the distal end of the collimatorholder 105 along an optical axis 104. As in FIGS. 1 and 2, thecollimator lens 103 is fitted in the distal end portion of thecollimator holder 105 at an accuracy at which the collimator lens 103 ispressed against the collimator holder 105. The slit 106 forms amechanical gap between the inner periphery of the collimator holder 105and the outer periphery of the collimator lens 103 and, at the sametime, makes the collimator holder 105 and the collimator lens 103mechanically rigid, thereby making the collimator lens 103 adjustable inthe direction of the optical axis 104. A rotation adjusting hole 107 isformed in a portion near the laser attached to the proximal end portionof the collimator holder 105. This rotation adjusting hole 107 functionsas an adjusting window in which an eccentric pin (to be described later)is inserted to rotate/adjust the astigmatic difference correctingoptical part 101 (to be referred to as an optical part hereinafter) suchas an inclined plate transparent glass. In the following description, inorder to simplify the description, this adjusting window is called as awindow portion. The adjusting unit 110 arranges the optical part 101between the laser beam source 102 and the collimator lens 103 androtates (rotational angle=φ) the optical part 101 around optical axis104 during astigmatic difference correction to adjust the optical part101.

FIG. 12 shows the embodiment figure of the adjusting unit 110 forastigmatic difference regulation in FIG. 1. As shown in FIG. 12, theoptical part 101 constituted by a cylindrical lens or an inclined platetransparent glass is rotated about the optical axis 104 between thelaser beam source 102 and the collimator lens 103 to correct theastigmatic difference of a focused beam. As described above, the opticalpart 101, e.g., an inclined plate glass 113, for correcting anastigmatic difference is incorporated in a cylindrical portion 112, and,as indicated by a portion surrounded by a broken line in FIG. 11, thecylindrical portion 112 is incorporated in the cylindrical portion 105in which the laser beam source 102 and collimator lens 103 constitutinga laser pen are integrated with each other. The circular window portion107 is formed in the cylindrical portion 105, and an eccentric pin 108is inserted in the window portion 107 to rotate/adjust the astigmaticdifference correcting optical part 101, e.g., the inclined plate glass113. A groove 109 in which the eccentric pin 108 is to be fitted isformed in the cylindrical portion 112 of the optical part 101 forcorrecting an astigmatic difference, and the astigmatic differencecorrecting cylindrical portion 112 is rotated about the optical axis 104with rotation of the eccentric pin 108. Rotational driving of theeccentric pin 108 is performed through a rotational driving actuator 111on the basis of the difference between the levels of an RF signal and atracking error signal (to be described later).

Returning to FIG. 11, the optical path changing unit 120 comprises acompound prism 121. The compound prism 121 comprises first and secondbeam splitters 122 and 123 arranged in the longitudinal direction of thecompound prism 121, and a 45° mirror portion 124 arranged at one end ofthe compound prism 121. In order to cause the compound prism 121 tochange the elliptic shape of a collimated beam emerging from thecollimator lens 103 into an almost circular shape, a beam shapingoptical portion (not shown) is arranged in the optical path directionchanging unit 120. The 45° mirror portion 124 is arranged to cause thecollimated beam to be incident on and emerge from the optical disk 150.In addition, the beam splitters 122 and 123 change the direction of anoptical path to detect a servo error signal and an RF signal.

Referring to FIG. 11 again, the spectral unit 130 comprises a dichroicmirror 131, a laser beam source 133, and an objective lens actuator 135.The laser beam source 133 comprises a hologram element 132. Theobjective lens actuator 135 comprises an objective lens 134. Thedichroic mirror 131 is arranged in the propagation direction of thecollimated beam raised by the 45° mirror portion 124. The dichroicmirror 131 causes a 680-nm beam to straightly propagate. The dichroicmirror 131 is optically designed such that when a beam from the laserbeam source 133 having a wavelength of 780 nm is incident on thedichroic mirror 131 from the direction perpendicular to the optical pathof the beam having a wavelength of 680 nm, the direction of the opticalpath of the beam is changed by 90°. The 780-nm beam reaches theobjective lens actuator 135, and is focused on the disk 150. Thedirection of the beam reflected by the disk 150 is changed by 90° by thedichroic mirror 131. A focusing error is detected independently of abeam from the 680-nm optical head, and the above objective lens actuator135 performs a focusing servo operation. More specifically, the opticalhead is controlled by the objective lens actuator 135 such that therelative, mechanical interval between the optical disk 150 and theoptical head changed by the outer shape and rotation of the optical disk150 is always kept constant.

In the example shown in FIG. 11, the hologram optical element 132 isincorporated in the 780-nm laser beam source 133. The laser beam source133 detects a focusing error signal when the beam reflected by the diskreturns to the laser beam source portion. A focusing error detectionmeans is constituted by the dichroic mirror 131, the hologram opticalelement 132, and the laser beam source 133.

In addition, a 680-nm collimated beam 125 emitted from the optical headis also focused on the optical disk 150, and the reflected beamreversely propagates on the optical path of the incident beam and isincident on the compound prism 121 to reach the second beam splitter 123for detecting the RF signal and the second beam splitter 122 fordetecting the tracking error signal.

FIG. 13 shows the spectral characteristics of the dichroic mirror 131shown in FIG. 11. As shown in FIGS. 11 to 13, the 680-nm laser beamsource 102 and the 780-nm laser beam source 133 for independentlyperforming the focusing servo operation for the objective lens 134 areimplemented in the optical head. The laser beam source 102 as the formeris designed to have a thin-film structure in consideration oftransmission characteristics, and the laser beam source 133 as the lateris designed to have a thin-film structure in consideration of reflectioncharacteristics. The transmittance of a beam having a wavelength=680 nm(arrow) is 95%, and the transmittance of a beam having a wavelength =780nm (arrow) is 5%. A reflectance is obtained by subtracting thereflectance (%) from 100%. In this manner, the independence of eachoptical system can be assured at a spectral ratio=19 (26 dB). Note thatthe dichroic mirror in FIG. 13 is designed such that the incident angleof a beam incident on a mirror plane is set to be 45°. However, when theincident angle is reduced to 40° and 30°, the dichroic mirror can bedesigned to decrease leakage beams in transmission and reflection.

FIG. 14 shows the optical arrangement of another spectral portionobtained such that the spectral characteristics of the dichroic mirror131 are improved. As shown in FIG. 14, a beam from the 780-nm laser beamsource 133 is temporarily incident on a dichroic mirror 136 having anincident angle=30° through a reflecting mirror 137. A beam reflected bythe dichroic mirror 136 is incident on the objective lens 134. With thisarrangement, the phase difference between P- and S-polarized componentscan be suppressed to be small, and this head can be satisfactorily usedas not only an optical head for recording/reproducing data on/from aphase-change medium but also a magneto-optical head.

Returning to FIG. 11 again, the error detector 140 comprises a λ/2 plate141 and a direction changing beam splitter 142 which are arranged on anoptical path of a beam whose direction is changed by the beam splitter123. In addition, a first error detection means is constituted by a pairof P- and S-polarizing photosensors 143 and 144 for detecting two typesof beams from the direction changing beam splitter 142 and a firstdifferential amplifier 145 electrically connected to the photosensors143 and 144. In addition, a first detection circuit 146 electricallyconnected to the first differential amplifier 145 is arranged.

The error detector 140 comprises a second error detection meansconstituted by a two-division photosensor 147 arranged on the opticalpath of a beam whose direction is changed by the second beam splitter122, and a second differential amplifier 148 connected to the outputterminals of the two-division photosensor 147. The error detector 140comprises a second detection circuit 149 connected to the second errordetection means. A third differential amplifier 151 is connected to theoutput terminals of the first detection circuit 146 and the seconddetection circuit 149, and the third differential amplifier 151 isconnected to a control device 160 to output an output from the thirddifferential amplifier 151 to the control device 160. The firstdetection circuit 146, the second detection circuit 149, and the thirddifferential amplifier 151 constitute a level determination means.

An operation of the error detector 140 will be described below.

In the first error detection means, a beam whose direction is changed by90° by the second beam splitter 123 is transmitted through the λ/2 plate141 to change the polarized direction of the beam, and the directionchanging beam splitter 142 splits the beam into two types of lightwaves, i.e., P- and S-polarized beams. These beams are received by theP- and S-polarizing photosensors 143 and 144, respectively, and outputsfrom the P- and S-polarizing photosensors 143 and 144 are supplied tothe first differential amplifier 145, thereby obtaining amagneto-optical RF signal from the first differential amplifier 145.

In the second error detection means, a beam changed by 90° by the secondbeam splitter 122 is input to the two-division photosensor 147, andtracking error detection using a push-pull scheme is performed. Atracking error signal is obtained by inputting two outputs from thetwo-division photosensor 147 to the second differential amplifier 148.In this case, when a focusing offset is changed, both the signals mustbe changed in the same manner.

For this reason, in the level determination means, in order to detectthe levels of the RF signal and the tracking error signal, the RF signaland the tracking error signals are supplied to the detection circuits146 and 149 comprising diodes 146a and 149a. Outputs from the detectioncircuits 146 and 149 are subtracted from each other by the thirddifferential amplifier 151 in an analog manner in FIG. 11. A resultobtained from the differential amplifier 151 is input to the controldevice 160.

The control device 160 and the actuator 111 constitute arotation/adjustment means. In the control device 160, a control signalcorresponding to the subtraction result from the third differentialamplifier 151, e.g., a signal for driving the actuator 111 in adirection to minimize the value of the subtraction result, is output tothe actuator 111. The actuator 111 is an electromechanical transducer,and rotates and adjusts the astigmatic difference correcting opticalpart 101 about an optical axis in accordance with the control signal tocorrect the astigmatic difference.

In this embodiment, adjustment of the optical part for correcting anastigmatic difference is performed through an electromechanicaltransducer having a control system and an actuator connected to thecontrol system. In this manner, in adjustment of a focusing offset forhigh-density recording, a best signal reproduction state can be obtainedunder optimal servo conditions. For this reason, a best carrier/noiseratio (CNR) can be obtained while stable servo characteristics are kept.

As has been described above, according to the present invention, afocusing offset for the best RF signal coincides with a focusing offsetfor the best tracking error signal. As a result, satisfactory servocharacteristics can be extracted, and preferable recording/reproducingcharacteristics can be obtained.

What is claimed is:
 1. An astigmatic difference correcting method for anoptical head, comprising the steps of:rotating and adjusting arotational optical member which is arranged between a first laser beamsource having an astigmatic difference and a collimator lens to berotatable around an optical path of a laser beam of a first wavelengthemitted from said first laser beam source; causing rotation andadjustment of said rotational optical member to set a minimum differencebetween focal positions in an X-axis direction and in a Y-axis directionwhich are perpendicular to the optical path of the laser beam;irradiating the laser beam in which the difference between focusingoffset positions is minimum on an optical disk having a recordingdensity having a bit length not more than half a laser wavelength;causing the laser beam of the first wavelength to be incident on andemerge from said optical disk through a dichroic mirror and an objectivelens; causing a laser beam of a second wavelength emitted from a secondbeam source to be incident on and emerge from said optical disk throughsaid dichroic mirror and said objective lens, said dichroic mirrorallowing one of the laser beams of the first and second wavelengths tobe transmitted through said dichroic mirror and allowing the other to bereflected by the dichroic mirror; and detected a focusing error betweensaid optical disk and said objective lens using a beam of the secondwavelength obtained through said dichroic mirror and reflected by saidoptical disk.
 2. An astigmatic difference correcting method for anoptical head, comprising the steps of:rotating and adjusting arotational optical member which is arranged between a first laser beamsource having an astigmatic difference and a collimator lens to berotatable around an optical path of a laser beam of a first wavelengthemitted from said first laser beam source; causing rotation andadjustment of said rotational optical member to set a minimum differencebetween focal positions in an X-axis direction and in a Y-axis directionwhich are perpendicular to the optical path of the laser beam;irradiating the laser beam in which the difference between focusingoffset positions is minimum on an optical disk having a recordingdensity having a bit length not more than half a laser wavelength;detecting a reproduction signal using a beam obtained by reflecting thelaser beam of the first wavelength by said optical disk; detecting atracking error signal of a push-pull scheme using the beam obtained byreflecting the laser beam of the first wavelength by said optical disk;and automatically controlling rotation and adjustment of said rotationaloptical member on the basis of levels of the reproduction signal and thetracking error signal.
 3. A method according to claim 2, wherein thestep of automatically controlling the rotation and adjustment of saidrotational optical member comprises the step of detecting thereproduction signal and the tracking error signal, the step ofdetermining levels of detection outputs of the reproduction signal andthe tracking error signal, and the step of rotating and adjusting saidrotational optical member in a direction to minimize a level differencebetween the two detection outputs.
 4. An astigmatic differencecorrecting apparatus for an optical head, comprising:a first laser beamsource, having an astigmatic difference, for emitting arecording/reproducing laser beam; a collimator lens, having acylindrical shape, for converting the laser beam from said laser beamsource into a parallel beam to output the parallel beam to an opticaldisk; rotational optical means, arranged between said laser beam sourceand said collimator lens to be rotatable around an optical path of thelaser beam, for causing rotation and adjustment to set a minimumdifference between focal positions in an X-axis direction and in aY-axis direction which are perpendicular to the optical axis of thelaser beam; a second laser beam source for emitting a laser beam of asecond wavelength; a dichroic mirror for causing the laser beams of thefirst and second wavelengths respectively emitted from said first andsecond beam sources to be incident on and emerge from said optical disk,said dichroic mirror allowing one of the laser beams of the first andsecond wavelengths to be transmitted through the dichroic mirror andallowing the other to be reflected by said dichroic mirror; and focusingerror detection means for detecting a focusing error between saidoptical disk and said objective lens using a beam of the secondwavelength obtained through said dichroic mirror and reflected by saidoptical disk.
 5. An astigmatic difference correcting apparatus for anoptical head, comprising:a first laser beam source, having an astigmaticdifference, for emitting a recording/reproducing laser beam; acollimator lens, having a cylindrical shape, for converting the laserbeam from said laser beam source into a parallel beam to output theparallel beam to an optical disk; rotational optical means, arrangedbetween said laser beam source and said collimator lens to be rotatablearound an optical path of the laser beam, for causing rotation andadjustment to set a minimum difference between focal positions in anX-axis direction and in a Y-axis direction which are perpendicular tothe optical axis of the laser beam; reproduction signal detection meansfor detecting a reproduction signal using a beam obtained by reflectingthe laser beam of the first wavelength by said optical disk; trackingerror signal detection means for detecting a tracking error signal of apush-pull scheme using the beam obtained by reflecting the laser beam ofthe first wavelength by said optical disk; and drive control means forperforming drive control for said rotational optical means on the basisof a reproduction signal level from said reproduction signal detectionmeans and a tracking error signal level from said tracking error signaldetection means.
 6. An apparatus according to claim 5, wherein saiddrive control means comprises first and second detection circuits forrespectively detecting the reproduction signal and the tracking errorsignal, a level difference detection circuit for detecting a leveldifference between outputs from said first and second detectioncircuits, and an actuator for driving said rotational optical means in adirection to minimize the level difference detected by said leveldifference detection circuit.
 7. An apparatus according to claim 5,further comprising a compound prism for splitting the beam obtained byreflecting the laser beam of the first wavelength by said optical diskinto beams toward said reproduction signal detection means and saidtracking error signal detection means.
 8. An astigmatic differencecorrecting apparatus for an optical head, comprising:a first laser beamsource, having an astigmatic difference, for emitting arecording/reproducing laser beam; a collimator lens, having acylindrical shape, for converting the laser beam from said laser beamsource into a parallel beam to output the parallel beam to an opticaldisk; a rotational optical device, arranged between said laser beamsource and said collimator lens to be rotatable around an optical pathof the laser beam, to cause rotation and adjustment to set a minimumdifference between focal positions in an X-axis direction and in aY-axis direction which are perpendicular to the optical axis of thelaser beam; a second laser beam source for emitting a laser beam of asecond wavelength; a dichroic mirror for causing the laser beams of thefirst and second wavelengths respectively emitted from said first andsecond beam sources to be incident on and emerge from said optical disk,said dichroic mirror allowing one of the laser beams of the first andsecond wavelengths to be transmitted through the dichroic mirror andallowing the other to be reflected by said dichroic mirror; and afocusing error detector for detecting a focusing error between saidoptical disk and said objective lens using a beam of the secondwavelength obtained through said dichroic mirror and reflected by saidoptical disk.
 9. An astigmatic difference correcting apparatus for anoptical head, comprising:a first laser beam source, having an astigmaticdifference, for emitting a recording/reproducing laser beam; acollimator lens, having a cylindrical shape, for converting the laserbeam from said laser beam source into a parallel beam to output theparallel beam to an optical disk; a rotational optical device, arrangedbetween said laser beam source and said collimator lens to be rotatablearound an optical path of the laser beam, to cause rotation andadjustment to set a minimum difference between focal positions in anX-axis direction and in a Y-axis direction which are perpendicular tothe optical axis of the laser beam; a reproduction signal detectordetecting a reproduction signal using a beam obtained by reflecting thelaser beam of the first wavelength by said optical disk; a trackingerror signal detector detecting a tracking error signal of a push-pullscheme using the beam obtained by reflecting the laser beam of the firstwavelength by said optical disk; and a drive controller performing drivecontrol for said rotational optical device on the basis of areproduction signal level from said reproduction signal detector and atracking error signal level from said tracking error signal detector.